Red fish under 500 SP
Minnow 500 SP
Dolphins 5000 SP
Orca 50K SP
Whale 500K SP
That is rough estimates since the levels are actually based upon MVests not SP, but it is close enough.
As for the loss of VP, it most certainly is one powering down, at least this move. This is going to happen from time to time. The second one you mentioned could be true. I havent look at the Whale list to see what percent of the Whales are bots. There is also the issue of delegation. I do not know what the system does with delegation. I believe if a Whale gives you 100K SP, it is still shown as the Whales, yet you benefit. So when @ned delegates a bunch of SP to Zappl (500K), he created another Whale but it is one that is using the SP to upvote users on that application..the same as D.Tube who got 1M SP.
So there are some things in place to spread the SP around.